As shown in FIG. 1, an electrostatic encoder 10 has, on a stator 11, a transmission electrode 12 and a detection electrode 13 and, on a mover 14 to be arranged at a position opposed to the electrodes, a relay electrode 15. A high-frequency signal 16, when being applied to the transmission electrode 12, is transmitted to the detection electrode 13 via a capacitance Ctc defined between the transmission electrode 12 and the relay electrode 15 and a capacitance Ccs defined between the relay electrode 15 and the detection electrode 13. Since the capacitance Ctc and the capacitance Ccs vary depending on the opposed correspondence of the transmission electrode 12, the relay electrode 15, and the detection electrode 13 based on the movement of the mover 14, a high-frequency signal emerging at the detection electrode 13 can be processed in a signal processing circuit 17 to detect the position of the mover 14. The principle of detecting the position of the mover using the electrostatic encoder is as follows.
With reference to FIG. 2, the high-frequency signal 16 is applied to the transmission electrode 12 of the electrostatic encoder 10. The high-frequency signal 16 causes the capacitance Ctc defined between the transmission electrode 12 and the relay electrode 15 to generate an electrostatically induced potential at the relay electrode 15, and the induced potential in turn causes the capacitance Ccs defined between the relay electrode 15 and the detection electrode 13 to generate a detection signal 18 at the detection electrode 13. Assuming that the capacitance Ctc between the transmission electrode 12 and the relay electrode 15 is fixed so as not to change, the capacitance Ccs changes as the mover 14 moves, so that the detection signal 18 has a waveform of amplitude modulation of the high-frequency signal 16. The signal processing circuit 17 can detect the amplitude-modulated signal component to calculate the position of the mover.
Based on the above-described fundamental principle of electrostatic encoder-based position detection, U.S. Pat. No. 4,429,307A (Patent Document 1) discloses an electrostatic encoder with transmission elements 56, 58 and reception elements 60 and conductive elements 50, 52 arranged, respectively, on a disk-shaped fixed disk 48 and a movable disk 46 (see FIGS. 7 and 8 of Patent Document 1). FIG. 3 shows the disk-shaped fixed disk 48 drawn in FIG. 8 of the U.S. patent. In the electrostatic encoder disclosed in the U.S. patent, a transmission signal (A sin ωt, −A sin ωt) transmitted from the transmission elements 56, 58 on the fixed disk 48 is relayed through the conductive elements 50, 52 to be detected at the reception elements 60. As the movable disk 46 rotates, the capacitance between the conductive elements 50, 52 and the reception elements 60 changes. The capacitance change is detected as a potential change, and two sinusoidally modulated output signals can thus be obtained with a mutual phase difference of 90 degrees. The amount of rotational displacement of the movable disk 46 can be detected from an envelope (amplitude modulation) component of the output signals.